This application relates to freeze dryers and, more particularly, to a method and apparatus for the construction of a shelf and tray for use in a freeze drying chamber.
Freeze drying is generally accomplished with different types of apparatus, e.g., manifold freeze dryers and bulk freeze dryers. Manifold freeze dryers are commonly used in small scale laboratory research, whereas bulk freeze dryers are for larger volumes of material to be dried. Generally, in the bulk freeze drying, the material to be dried is poured directly into trays or into individual containers which are then placed on trays. The trays are then placed inside one large vacuum chamber, which is automatically refrigerated and to which vacuum is applied.
A fast and efficient freeze drying process requires that the material being dried be first frozen in an efficient manner. Thus, thermal conductivity between the tray and the environment in which the tray sits must necessarily be very high. To attain such high thermal conductivity, the shelf and the tray should be in intimate physical contact at as many points as possible. This may be accomplished in one manner by clamping the tray to the shelf at various points on the periphery. However, this procedure is time consuming, and the time spent in clamping the tray to the shelf might just as efficiently be spent in additional refrigeration time of the tray. Therefore, a need has arisen for a means of quickly and efficiently locking a tray containing the material to be freeze dried onto the shelf to gain as much surface contact between the shelf and the tray as possible.
Even though the tray may be clamped onto the shelf, this alone does not insure the maximum surface contact of the tray and the shelf. A tray may be made to conform to a substantially flat shelf surface when locked into place on the shelf. However, if the shelf presents an irregular or warped surface, surface contact, and thus thermal conductivity, between the shelf and the tray is reduced. Thus, a method of constructing a substantially flat shelf, while retaining the fluid-tight characteristics of the shelf for refrigeration purposes, becomes desirable.
The bulk freeze dryer generally comprises a freeze drying chamber having one or more shelves. These shelves generally have condenser coils soldered or welded to its underside to provide the cooling capacity of the freeze dryer. The condenser coils are generally welded to the shelf to provide intimate contact with the shelf for efficient thermal transfer. However, the problem arises in the welding of the condenser tubing to the shelf surface that the shelf surface itself warps, thereby losing its effectiveness as a thermal transfer surface as there is no longer the maximum surface contact at the interface of the tray with the shelf surface.